/**
 * @a https://leetcode.cn/problems/reverse-nodes-in-k-group/
 */

#include "common.h"


struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        int n = 0;
        ListNode *cur = head;
        while(cur){
            cur = cur->next;
            ++n;
        }
        n /= k;

        ListNode* newHead = new ListNode(0);
        ListNode *prev = newHead;
        cur = head;
        for(int i = 0; i < n; ++i){
            ListNode *tmp = cur;
            for(int j = 0; j < k; ++j){
                ListNode* next = cur->next;
                cur->next = prev->next;
                prev->next = cur;
                cur = next;
            }
            prev = tmp;
        }
        prev->next = cur;
        
        cur = newHead->next;
        delete newHead;
        return cur;
    }
};